a hole is not really going to mess up the barrel use a bolt and screw it into the side if you are pissed of at the thing not being smooth like the rest of the barrel you can patch it with some epoxy or grind it with a dremel tool

Maybe you could use a 4 inch long section of half inch PVC and then drill a hole through that half inch piece and then just ziptie it to the pipe? I don't know, I say epoxy or JB Weld if all else fails.

Use epoxy. Just put a dollop of it at the head of the nail, then put it onto the barrel, then once it has hardened, put some more around the base, it will hold.

Or alternatively, you could get some fencing wire, and bend it into a "u" shape, with one end of the "u" coming up and out to from the handle. The epoxy will have heaps of surface area to grip the the metal, creating a strong hold. i did this with hot glue and it worked well.